The present document is based on Japanese Priority Document JP2002-014149, filed in the Japanese Patent Office on Jan. 23, 2002, the entire contents of which being incorporated herein by reference.
1. Field of the Invention
The present invention relates to an antenna apparatus including an array antenna used in a time division multiple communication system and its signal calibration circuit and particularly to an antenna apparatus capable of calibrating an amplitude of a transmission/reception signal of each antenna element and a phase thereof without having to depend on the outside information.
2. Description of Related Art
In a time division multiple communication system using an array antenna, signals are send to each antenna element during transmission. However, even if the same signals are send to each antenna element, errors are generated in an amplitude and a phase of the signal to be emitted by each antenna element according to variations of a gain characteristic of an amplification circuit for amplifying the output of a transmitter (a transfer function), and an amplitude characteristic and a phase characteristic of a cable for propagating a signal, a filter for controlling a frequency component of the signal and an antenna element. Due to these errors, the control of a transmission beam pattern becomes difficult, so that beam forming can not be correctly realized.
In addition, also during reception, the similar problems may occur. When a signal arrives at each receiver, an output signal of the receiver has errors in the amplitude and the phase of the signal received by each antenna element depending on variations of a filter characteristic, a gain characteristic of the amplification circuit, and further, a character of a cable for propagating the received signal or the like. Thus, there may be a problem such that a direction of arrival (DOA) of the received signal can not be correctly estimated or the like.
Therefore, in an antenna apparatus using the array antenna, a measure is taken that a calibration circuit is provided to obtain a calibration factor for correcting the error of each transmitter-receiver.
FIG. 6 is a configuration example of an entire antenna system including a conventional calibration circuit. As shown in the drawing, this antenna apparatus is constructed of an antenna array including antenna elements 11, 12, 13-1L, filters 21, 22, 23-2L connected to each antenna element, antenna switches 31, 32, 33-3L connected to each filter, a calibration circuit 200, transmitters 81, 82, 83-8L and receivers 91, 92, 93-9L.
The calibration circuit 200 is constructed of directional couplers 41, 42, 43-4L and SPDT (single pole double through) RF switches (hereinafter, as a matter of convenience, simply referred to as a RF switch) 51, 52, 53-5Lxe2x88x921, 62, 63-6Lxe2x88x921, 71, 72, 73-7L.
The directional couplers 41, 42-4L output the inputted signals to other output terminals with a predetermined amplitude attenuation rate and a predetermined phase difference.
The antenna switch has three terminals and is capable of switching between the transmitting signal and the received signal. For example, the antenna switch 31 has terminals a1, a2 and a3, and the terminals a1 and a2 are connected during transmission so that a signal is sent from the directional coupler 41 to a filter 21. In addition, during reception, the terminal a1 is connected to the terminal a3, so that the received signal from the filter 21 is sent to the directional coupler 41.
In addition, each RF switch also has three terminals, respectively. For example, the RF switch 71 has terminals b1, b2 and b3. The terminal b1 is connected to the terminal b2 during the reception and the terminal b1 is connected to the terminal b3 during calibration.
During the normal transmission, the signal to be sent from the transmitter 81 is inputted to the terminal a2 of the antenna switch 31 through the directional coupler 41. In this time, since the terminal a2 is connected to the terminal a1 in the antenna switch, the signal is inputted to the filter 21. Then, a signal component that passed through the filter 21 is transmitted to the antenna element 11, and emitted in the air.
In addition, in the same way, the signal to be sent from the other transmitter 82, 83-8L are transferred to the antenna elements 12, 13-1L, respectively, to be emitted in the air from each antenna element. During the transmission, by controlling the phase of the signal to be transmitted to each antenna element, a predefined beam pattern is formed, namely, a so-called beam forming can be realized.
The beam forming during the transmission can be realized, for example, by outputting a signal to each antenna element, in which the signals from the transmitters 81, 82, 83-8L are multiplied by predefined weighting factors for controlling an amplitude and a phase, respectively. In addition, the weighting processing, which multiplies the transmitting signal by this weighting factor, can also be realized, for example, by a multiplier provided between the antenna element and the filter, or between the filter and the antenna switch.
During the normal reception, the signal received by the antenna element 11 is transmitted to the filter 21 and then, the signal component that passed through the filter 21 is inputted to the terminal a1 of the antenna switch 31. In this time, since the terminal a1 is connected to the terminal a3 in the antenna switch 31, and then the received signal is transmitted to the terminal b2 of the RF switch 71. The terminal b2 is connected to the terminal b1 in the RF switch 71, so that the received signal is inputted to the receiver 91. Further, in the same way, the received signals of the other antenna elements 12, 13-L1 are transferred to the receivers 92, 93-9L, respectively.
In addition, not only during the transmission but also during the reception, by way of multiplying the received signal from each antenna element by a predefined weighing factor, respectively, the beam forming can be realized. Further, by way of measuring the received signal while sweeping the received beam during the reception, it is also possible to estimate an arrival direction of the received signal.
According to the above described antenna apparatus, when calibrating the device, on the basis of a transmitter and a receiver, it is possible to obtain the calibration factors of respective transmitters and receivers by measuring the amplitudes changes and the phase changes of the transmitting signals and the received signals of the other transmitters and receivers with respect to the above mentioned reference transmitter and receiver.
With reference to FIG. 7, a method of the calibration in the antenna apparatus shown in FIG. 6 will be described below.
FIG. 7 is a conceptual diagram for showing a flow of a signal when calibrating the antenna apparatus. In FIG. 7, a transmission/reception set including a transmitter 8ixe2x88x921 (i=2,3-L) and a receiver 9ixe2x88x921 and a transmission/reception set including a transmitter 8i and a receiver 9i are depicted.
As shown in FIG. 7, the calibration for the transmitter 8i and the receiver 9i is carried out by measuring a received signal Tixe2x88x921 Ri of the receiver 9i when transmitting a signal from the transmitter 8ixe2x88x921 and by measuring a received signal Ti Rixe2x88x921 of the receiver 9ixe2x88x921 when transmitting a signal from the transmitter 8i. 
A principle of the calibration and how to obtain the calibration factor will be described below.
According to the antenna apparatus shown in FIG. 6, in the receivers 91 to 9L, the optimum weighting factors are defined as W01 to W0L, such that there is no amplitude difference and no phase difference between the received signals r1 to rL. The output Yri of the receiver 9i is represented as follows.
[Equation 1]
Yri=W0iSri=WiMiRiSrixe2x80x83xe2x80x83(1) 
In the equation (1), Sri represents a received signal of the antenna element 1i and Wi represents a weighting factor that is appropriately used for the received signal during the reception. Further, this weighting factor W1 is multiplied by the received signal in analog or in digital at inside or outside of each receiver. In addition, Mi represents a complex number of a coefficient showing the amplitude change and the phase difference, which are generated in the antenna, the filter and a cable on a path for propagating the signal from the antenna element to the receiver 9i. 
On the other hand, if a signal to be transmitted from the transmitter 8i is defined as Sti, a signal yti that was emitted from the antenna element 1i in the air after controlling the directionality can be calculated on the basis of the following equation.
[Equation 2]
Yti=WiMiTiStixe2x80x83xe2x80x83(2) 
In the equation (2), Ti is a transfer function to indicate a total gain of an amplification circuit and a filter or the like that are provided inside or outside of the transmitter 9i. Normally, this transfer function Ti is a complex number including the both of the amplitude component and the phase component.
In order to harmonize the antenna patterns at the transmission/reception, the following equation should be established in the antenna apparatus.
[Equation 3]
Yti=Yrixe2x80x83xe2x80x83(3) 
If the equations (1) and (2) are assigned into the equation (3), the following equation can be obtained.
[Equation 4]
Yti=(W0i/MiRi)MiTiSri=W0i(Ti/Ri)Srixe2x80x83xe2x80x83(4) 
Here, the following equation is presumed.
[Equation 5]
Di=Ti/Rixe2x80x83xe2x80x83(5) 
As a coefficient for correcting Di, a coefficient Ci is obtained as represented by the following equation, because Di is different in each antenna element.
[Equation 6]
Ci=Ri/Tixe2x80x83xe2x80x83(6) 
If based on a signal to be transmitted from the antenna element 11, a calibration factor Hi for correcting a signal to be transmitted from the antenna element 1i is obtained by the following equation.
[Equation 7]
Hi=Ci/C1=(T1Ri/TiR1)xe2x80x83xe2x80x83(7) 
In the next place, the operation for measuring the calibration factor is described below.
As shown in FIG. 6, in the RF switch 51, the terminal c1 is connected to the terminal 3 of the directional coupler 42, and the terminal c2 is connected to the terminal b3 of the RF switch 71.
In the RF switch 62, the terminal d1 is connected to the terminal b3 of the RF switch 72, and the terminal d2 is connected to the terminal 3 of the directional coupler 41.
In the beginning, when calibrating the signal to be transmitted from the antenna element 12, the terminals a1 and a2 are connected in the RF switches 31 and 32, respectively. The terminals d1 and d2 of the RF switch 62 are connected, and the terminals b1 and b3 of the RF switch 72 are connected to each other. Further, the terminals c1 and c2 of the RF switch 51 are connected, and the terminals b1 and b3 of the RF switch 71 are connected to each other.
Under such a condition, a signal T1 from the transmitter 81 is inputted to the terminal 1 of the directional coupler 41. The signal is coupled from the terminal 1 to the terminal 3 in the directional coupler 41 to be inputted to the terminal d2 of the RF switch 62, and then, this signal is inputted to the receiver 92 via RF switch 72. At this point, the receiver 92 measures the amplitude and the phase of the signal as a received signal T1R2.
In the same way, a signal T2 from the transmitter 82 is inputted to the terminal 1 of the directional coupler 42. The signal is coupled from the terminal 1 to the terminal 3 in the directional coupler 42 to be inputted to the terminal c1 of the RF switch 51, and then, this signal is inputted to the receiver 91 via RF switch 71. In this case, the receiver 91 measures the amplitude and the phase of the signal as a received signal T2R1.
In the same way, in order to obtain the calibration factor for correcting a signal to be transmitted from the antenna element 1i, as shown in FIG. 7, in the antenna switches 3ixe2x88x921 and 3i, the terminals a1 and a2 are connected to each other, respectively. Then, the transmitters 8ixe2x88x921 and 8i transmit signals Tixe2x88x921 and Ti, respectively.
When the transmitter 8ixe2x88x921 transmits the signal Tixe2x88x921, the signal coupled from the terminal 1 to the terminal 3 in the directional coupler 4ixe2x88x921 is sent to the receiver 9i via an RF switch that is not shown in FIG. 7. Then, the receiver 9i measures the amplitude and the phase of the signal as a received signal Tixe2x88x921 Ri.
In the same way, when the transmitter 8i transmits the signal Ti, the signal coupled from the terminal 1 to the terminal 3 in the directional coupler 4i is sent to the receiver 9ixe2x88x921 via an RF switch that is not shown in FIG. 7. Then, the receiver 9ixe2x88x921 measures the amplitude and the phase of the signal as a received signal Ti Rixe2x88x921.
Thus, the calibration factor for correcting a signal to be transmitted from the antenna element 1i can be obtained from the following equation.
[Equation 8]                                                                        H                i                            =                                                C                  i                                /                                  C                  1                                                                                                        =                              (                                                      T                    1                                    ⁢                                                            R                      i                                        /                                          T                      i                                                        ⁢                                      R                    1                                                  )                                                                                        =                                                                    T                    1                                    ⁢                                                            R                      2                                        ·                                          T                      2                                                        ⁢                                      R                    3                                                  -                                                      T                                          i                      -                      1                                                        ⁢                                                            R                      i                                        /                                          T                      2                                                        ⁢                                                            R                      1                                        ·                                          T                      3                                                        ⁢                                      R                    2                                                  -                                                      T                    i                                    ⁢                                      R                                          i                      -                      1                                                                                                                              (        8        )            
However, in the conventional signal calibration circuit and the antenna apparatus using it, an error may be generated in the calibration factor that is obtained by the above described calibration method. For example, according to the calculation of a calibration factor Hi on the basis of the equation (8), the amplitude change and the phase difference between the RF switches composing a signal correction circuit shown in FIG. 6 are not considered, so that the amplitude change and the phase difference there between may be included in the calibration factor as an error. In order to accurately correct the error, a layout of the wiring between the RF switches may be restricted.
In addition, according to the above described method for obtaining the calibration factor, since a received signal Ri of each antenna element 1i is not directly measured, W0i of each receiver is not possible to be directly obtained. Accordingly, this involves a disadvantage such that, the arrival direction of the signal received by antenna elements may not be estimated on the basis of the obtained calibration factor.
The present invention is conceived taking the foregoing problems into consideration and provides an antenna apparatus capable of measuring a calibration factor accurately and capable of estimating an arrival direction of a received signal by configuring a calibration circuit using a directional coupler.
In order to solve or alleviate the above mentioned problems, an antenna apparatus according to an embodiment of the present invention may include: a first branch unit including a first antenna element, a first transmitter, and a first directional coupler for outputting a received signal from the first antenna element to an output terminal during the reception, outputting a transmitting signal from the first transmitter to the first antenna element during the transmission, and providing feedback of the transmitting signal from the first transmitter to the output terminal with a predetermined attenuation rate; a second branch unit including a second antenna element, a second transmitter, and a second directional coupler for outputting a received signal from the second antenna element to an output terminal during the reception, outputting a transmitting signal from the second transmitter to the second antenna element during the transmission, and providing feedback of a transmitting signal from the second transmitter to the output terminal with a predetermined attenuation rate; a third directional coupler for outputting an inputted signal of a first input terminal to a first output terminal during the reception, coupling the inputted signal to a second output terminal with a predetermined attenuation rate, outputting an inputted signal of a second input terminal to a second output terminal, and coupling the inputted signal to a first output terminal with a predetermined attenuation rate, wherein the first and the second input terminals are connected to directional couplers at the first branch unit and the second branch unit, respectively, and the first and the second output terminals are connected to the first and the second receivers, respectively; and operating means for obtaining a calibration factor for calibrating the second branch unit in accordance with a first received signal for calibrating, which is coupled to the second receiver by the first and third directional couplers when the first transmitter transmits the signal, and a second received signal for calibrating, which is coupled to the first receiver by the second and third directional couplers when the second transmitter transmits the signal.
According to another embodiment of the present invention, it is preferable that the operating means obtains a calibration factor to be applied to the second branch unit in accordance with a ratio between the first received signal and the second received signal using the first branch unit as a reference.
According to still another embodiment of the present invention, it is preferable that in the third directional coupler, an attenuation rate in the case that the inputted signal of the first input terminal is sent to the second output terminal and an attenuation rate in the case that the inputted signal of the second input terminal is sent to the first output terminal are equally formed.
The antenna apparatus according to another embodiment of the present invention may include: L-number branch units (L=2, L is an integer) having antenna elements, directional couplers and transmitters, wherein the directional couplers output the received signal from the antenna element to an output terminal during the reception, output the transmitting signal from the transmitter to the antenna element during the transmission, and provides feedback of the transmitting signal from the transmitter to the output terminal with a predetermined attenuation rate; a plurality of directional couplers at a second layer, each of which is provided for each pair of the L-number branch units, wherein a first input terminal is connected to the directional coupler at the ith branch unit (1=i=L, i is an odd number), a second input terminal is connected to the directional coupler at the (i+1)th branch unit, an inputted signal from the first input terminal is sent to a first output terminal, an inputted signal from the second input terminal is coupled to a first output terminal with a predetermined attenuation rate, and further, an inputted signal from the second input terminal is sent to second output terminal, and an inputted signal from the first input terminal is coupled to the second output terminal with a predetermined attenuation rate; a plurality of directional couplers at a third layer, which are provided to the directional couplers at the second layer on a one-to one basis, wherein first and second input terminals are connected to a first output terminal or a second output terminal of the adjoining directional coupler at the second layer, and the receiver is connected to the first and second output terminals; and operating means for obtaining a calibration factor at the each branch unit in accordance with the received signals of the (ixe2x88x921)th and (i+1) th receivers when the transmitter at the ith branch unit transmits a signal.
According to the embodiment of the present invention, it is preferable that the directional coupler at the third layer outputs the inputted signal of the first input terminal to the first output terminal, and couples the inputted signal to the second output terminal with a predetermined attenuation rate, outputs the inputted signal of the second input terminal to the second output terminal, and at the same time, couples the inputted signal to the first output terminal with the predetermined attenuation rate.
According to the embodiment of the present invention, it is preferable that, assuming that, when the transmitter at the ith branch unit transmits a signal, the received signal at the (ixe2x88x921)th receiver is defined as TiRixe2x88x921, and when the transmitter at the (ixe2x88x921)th branch unit transmits a signal, the received signal at the ith receiver is defined as Tixe2x88x921Ri, the operating means calculates a calibration factor Hi at the ith branch unit on the basis of the first branch unit by the following equation, namely,                               H          i                =                ⁢                              T            1                    ⁢                                    R              i                        /                          (                                                T                  i                                ⁢                                  R                  1                                            )                                                              =                ⁢                                            T              1                        ⁢                                          R                2                            ·                              T                2                                      ⁢                          R              3                                -                                    T                              i                -                1                                      ⁢                                          R                i                            /                              T                2                                      ⁢                                          R                1                            ·                              T                3                                      ⁢                          R              2                                -                                    (                                                T                  i                                ⁢                                  R                                      i                    -                    1                                                              )                        .                              
As described above, it is possible to easily obtain a calibration factor for calibrating variations of a transfer function of a transmitter and a receiver and a signal propagation characteristic of each branch unit by providing the directional couplers, which are formed symmetrically in a structure, at respective branch units.
Further, there is an advantage such that it is possible to calculate the optimum coefficient at each branch unit, respectively, and by thus, an arrival direction of a received signal can be estimated.